Ferritic alloy for constructions

ABSTRACT

A compound tube including a layer of a Fe-Cr alloy and a layer of load-carrying component, with the optional addition of further layers. The Fe-Cr alloy preferably has a composition including, in weight-%: less than 0.3% carbon, 15-60% chromium, less than 10% nickel, less than 5% molybdenum, less than 5% silicon, less than 0.3% nitrogen, less than 5% manganese, and the remainder iron with naturally occurring impurities. The compound tubes are very resistant to carbonization and metal dusting, and are suited for use as bayonet tubing, superheater tubing, and reforming tubing in steam formation environments.

The present invention relates to the use of a ferritic iron chromiumalloyed construction material for the production of multi layeredcompound tubes, which must meet the demands for good resistance againstoxidation, carburisation and so called “metal dusting” in applicationssuch as bayonet tubes, superheater and reformer tubes in steam reformingplants. The invention also relates to the compound tubes per se. Such anouter or inner material component is especially advantageous in aco-extruded tube, where the inner, alternatively the outer, materialcomponent consists of a conventional steel or a nickel base alloy withgood strength.

With compound tube is intended a tube consisting of two layers with socalled metallurgical bonding between the components. Metallurgicalbonding is necessary in order to maintain a good thermal conductivity.The compound tube is made by so called co-extrusion.

Steam reforming means the process steps for the production of so calledsynthesis gas, for generation of for instance ammonia, methanol andhydrogen gas, where water vapour is mixed with hydrocarbons in order toform hydrogen gas and carbon oxide. With reformer tubes are meant thecatalyst filled tubes in which steam and hydrocarbons are convertedwholly or partially to hydrogen gas and carbon oxide at hightemperatures. Bayonet tubes are in this context a type of tubes placedinside the reformer tubes and function as a heat exchanger as theprocess gas which flow through them emits its heat to the gas whichflows on the outside. Superheater tubes are placed, in the form of coilsor cores, after the reformer and are used in order to cool down theprocess gas by super heating of steam.

The solutions, which are used today for bayonet tubes, superheater andreformer tubes, where metal dusting constitutes a problem, are generallynickel base alloys or stainless steels. However, these materials havelimited resistance against metal dusting, which gives a shortened lengthof life or results in that non optimum process parameters for theexchange must be used in the steam reforming. The nickel base alloys arefurther very expensive due to high amounts of alloying elements anddemanding manufacturing processes.

A first aim with the present invention is consequently to develop a moreresistant product to a lower cost than the present solutions. This aimhas been achieved by using alloys having a composition according to thepresent invention in the production of compound tubes.

The production of the compound tube is done in a way that the twodifferent components are made to bars in a conventional manner. The barsare drilled and turned with close tolerance demands and are put togetherto a co-extrusion blank. The corrosion protective ferritic iron chromiumalloyed material usually constitutes between 20-50% of the total wallthickness.

The blank is heated to a temperature between 900 and 1200° C. and isco-extruded into a tube. The co-extruded tubes cools in air in order tominimise bent tubes due to thermal tensions created during the cooling.Cold working operations (cold rolling) to finished dimension follows ifnecessary.

During the co-extrusion process the metallurgical bonding is created.This, like the layer thickness of both the components; is attested bymeans of a control of the finished product ready for delivery.

The present invention is based on the discovery that compound tubes inspecific alloying combinations can fulfil all the demands set onconstruction materials intended to be used as bayonet tubes andsuperheater and reformer tubes in steam reforming plants. The demandsthat must be satisfied are good resistance against metal dusting,oxidation resistance, sufficient mechanical properties (as strength) andstructure stability.

Testing in laboratory scale and in production plants has shown that theferritic iron chromium alloy is superior to the materials normally usedtoday in steam reforming plants. Previously known materials aredescribed in for instance Stahl and Thomsen: Survey of WorldwideExperience with Metal Dusting, presented at the AIChE symposium onammonia safety, Tucson, Ariz., Sept. 18-20, 1995; Grabke, Krajak andMuiller-Lorenz: Werkstoffe und Korrosion 44:89-97 (1993), andRichardson: Nitrogen No. 205, September-October 1993.

The invention includes the use of an iron chromium alloy with ferriticstructure and containing, in weight-%:

LEVEL¹ 1 2 3 carbon <0.05 <0.10 <0.3 chromium 20-30 15-40 15-60 nickel<2 <10 molybdenum <2 <5 silicon <2 <5 nitrogen <0.05 <0.10 <0.3manganese <2 <5 iron rest (except usual impurities) ¹Level 3: suitablecontent Level 2: preferred content Level 1: specially preferred content

The alloy above will constitute the, for corrosion by metal dusting andcarburisation, exposed component in a compound tube made byco-extrusion, where the other, load carrying component consists of alower alloyed carbon steel, a so called 9-12% chromium steel, aconventional stainless steel or a nickel base alloy. Which of thecomponents is the outer or inner component depends on if the process gasflows on the in- or outside of the tube.

The environments where metal dusting and carburisation arise arecharacterised by a high carbon activity and a relatively low oxygenpartial pressure in the process gas, and a normal temperature of450-900° C. In order to be resistant against this type of corrosion ametallic material is required to have a good ability to form aprotective oxide on the surface. Decisive for this ability is mainly thecontent of the oxide forming element in the material and the microstructure of the material. Due to the relatively low oxygen content inthe gas, only three types of protective oxides can practically be formedin the actual environment: aluminium oxide, chromium oxide and silica.Steel alloys or nickel base alloys with aluminium or silicon in order topromote formation of these types of oxides result in deterioratedductility of the alloy, which makes the making very difficult. Thediffusion of the oxide forming element to the surface is critical, whyit is a prerequisite in the actual temperature region that the alloy hasa micro structure with a ferritic matrix.

The ferritic iron chromium alloyed material of the invention has on theother hand very low strength at high temperatures and can also beembrittled during operation by formation of so called sigma phase. It istherefore not suitable for use in applications that work undermechanical stress. The low strength makes further that it is easilydeformed by creep, which is negative for instance for protection againstmetal dusting, since the protective oxide is easily broken up. Thatmeans that the ferritic iron chromium alloyed material as such can notbe used as bayonet tubes, superheater and reformer tubes in steamreforming plants.

The joining of corrosion resistant ferritic iron chromium alloyedmaterial, which usually constitutes 20-50% of the total wall thickness,with an alloy with high strength in the form of a compound tube, so thatthat the iron chromium alloyed material is exposed to the corrosiveprocess gas, a product that manages both the demands on resistance tometal dusting and mechanical hot strength is obtained. The tubes mayhave an outer diameter of 15-200 mm and a total wall thickness of 2-20mm.

The choice of load carrying component, i e the component on which theferritic iron chromium alloyed corrosion protection steel shall beapplied, depends on the working temperature and the mechanical stress ofthe component. Besides demands on strength there are demands onresistance against oxidation in combustion gases or water vapour for theload carrying component. It can generally be said that the oxidationproperties become more decisive the higher the working temperature ofthe component. Oxidation resistance is generally achieved by alloyingwith chromium. Suitable alloys for the load carrying component aretherefore at higher(T≧550° C.) temperatures, austenitic stainless steelsor Ni—Cr-alloys. At lower temperature (T≦600° C.) lower alloyed steels,so called 9-12% chromium steels might be suitable as load carryingcomponent.

An example of a suitable load carrier for the type of compound tube, tobe used at temperatures above 600° C., is Alloy 800H(Fe-30Ni-20Cr-0.4Al-0.4Ti). It is characterised by good creep strengthand structure stability, which makes it suitable for use in pressurisedapplications. It has further a good oxidation resistance, which makes itresistant to for instance combustion gases.

An example of a suitable load carrier for the type of compound tube, tobe used at temperatures below 600° C., is alloy SS142203(Fe-0,15C-9Cr-1Mo). It is characterised by good hot strength and isapproved for use in pressurised applications. It has further a goodoxidation resistance, which makes it resistant to for instancecombustion gases at the actual temperature.

Below follows a short report on the influence of each element in thefinal steel alloy. The influence of the elements does obviously decidethe desired min.- and max.-contents according to the level overviewabove.

C, N: too high carbon/nitrogen content has a negative influence on theload carrying component. Carbon/nitrogen diffuses into it duringoperation, which results in a deteriorated ductility (brittleness).

Cr: the chromium content should be >5%, and preferably >15%, in order tomake formation of protective chromium oxide possible. Too high chromiumcontent results in great working problems.

Ni: nickel is austenite stabilising, i e at too high content the matrixis no longer ferritic, which is a prerequisite in order to form aprotective oxide layer. Ni is further an expensive alloying element andshould therefore be kept low.

Mo: high content of Mo can result in formation of a melted oxide at hightemperatures, which reduces the metal dusting resistance of thematerial. Mo is also expensive.

Mn: is like nickel austenite stabilising, i e at too high content thematrix is no longer ferritic, which is a prerequisite in order to form aprotective oxide layer.

Si: too high silicon content is embrittling and results in great workingproblems.

EXAMPLE 1

A steel melt with composition A (the alloy according to the invention)according to table 1 was made in a conventional way by melting of scrapin an electric arc furnace, refining and decarburisation in an AODconverter and continues casting to size 265×265 mm. The continuouslycasted blank was then hot rolled to round bar of size Ø144 mm. From thisbar a 520 mm long blank was cut, which was turned to an outer diameterof 120 mm, and in which a Ø93 mm through hole was drilled.

A steel melt with composition B, which is intended for the load carryingcomponent, according to table 1, was made in the same manner as melt A,but was instead hot rolled to round bar of size Ø120 mm. From this bar a520 mm long blank was cut, which was turned to an outer diameter of 93mm, and in which a Ø45 mm through hole was drilled.

The two inserts were joined by placing the blank of melt B inside theblank of melt A, whereafter the two components were co-extruded at 1100°C. to a tube with an outer diameter of 48 mm and a wall thickness of 4.5mm. The tube was annealed and then step rolled to outer diameter Ø31.8and wall thickness 3.0 mm.

The finished tube had an outer component with the thickness 1.17-1.30 mmand an inner with the thickness 1.54-1.74 mm. These variations arenormal and totally acceptable.

Ring widening testing, which means that short tube pieces are widened bybeing pressed down on a conical mandrel that forces the ring (the tubepiece) to expand, was performed to check the ductility of the tubes. Alltested rings passed the demand for 20% widening.

Flattening test, which means that cut tube pieces are pressed togetherin radial direction, was also performed to check the ductility of thetubes. All tested tube pieces managed flattening from 31.8 to 9 mm,which is approved.

Tensile testing of complete tube sections, i e both components, resultedin:

Yield point Rp0.2: 352-380 MPa Rupture limit Rm: 573-592 MPa Ruptureelongation A50: 40.2-42.6%

Since the two components have different strength, these values areconstituted by a mean value of the contribution from each component. Thevalues can be considered normal and shows that the finished tube hasapproved properties.

TABLE 1 (values in weight %) Charge % C % Si % Mn % Cr % Ni % Mo % Al %Ti % N A 0.19  0.44 0.88 26.44  0.43 0.06 0.003 0.01 0.16  B 0.065 0.430.56 20.80 31.09 0.03 0.45  0.40 0.017

EXAMPLE 2

A steel melt with composition C according to table 2 was made in aconventional way by melting of scrap in an electric arc furnace,refining and decarburisation in an AOD converter and continues castingto size 265×265 mm. The continuously casted blank was then hot rolled toround bar of size Ø185 mm.

From this bar a 775 mm long blank was cut, in which a Ø117 mm throughhole was drilled.

A steel melt with composition D, which is intended for the load carryingcomponent, according to table 2, was made in the same manner as melt A,but was instead hot rolled to round bar of size Ø138 mm. From this bar a775 mm long blank was cut, which was turned to an outer diameter of 117mm, and in which a Ø81 mm through hole was drilled.

The two inserts were joined by placing the blank of melt D inside theblank of melt C, whereafter the two components were co-extruded at 1100°C. to a tube with an outer diameter of 89 mm and a wall thickness of 7.5mm. The tube was annealed and then step rolled to outer diameter 63 andwall thickness 5.0 mm. By this, a finished tube with an outer componentwith the thickness 3.6 mm and an inner with the thickness 1.4 mm, wasachieved.

The finished tube was tested with non destructive ultrasonic soundtesting. No defects, for instance in the so called bonding zone betweenthe inner and outer component, were found.

TABLE 2 (values in weight %) Charge % C % Si % Mn % Cr % Ni % Mo % Al %Ti % N C 0.17  0.59 0.78 26.70 0.26  0.02 0.003 0.01 0.18  D 0.067 0.590.53 20.15 30.20 0.08 0.46  0.53 0.014

The compound tubes made according to the invention has an, until nowunattained, resistance to petal dusting and a long adequate length oflife.

What is claimed is:
 1. A method of use of an iron chromium alloy with aferritic microstructure, the iron chromium alloy includes in weight-%:carbon: <0.3, chromium: 15-60, nickel: <10, molybdenum: <5, silicon: <5,nitrogen: <0.3, manganese: <5, and the rest iron, apart from naturallyoccurring impurities; wherein the method comprises forming a compoundtube comprising a layer of a load carrying material and a layer of theiron chromium alloy.
 2. Use according to claim 1, where the chromiumcontent is between 15 and 40 weight-%.
 3. Use according to claim 1,where the nickel content is <2 weight-%.
 4. Use according to claim 1,where the silicon content is <2 weight-%.
 5. Use according to claim 1,where the nitrogen content is <0.10 weight-%.
 6. Compound tube includingat least one layer of an iron chromium alloy, and at least one layer ofa load carrying component, wherein the iron chromium alloy has thefollowing composition in weight-%: carbon: <0.3, chromium: 15-60,nickel: <10, molybdenum: <5, silicon: <5, nitrogen: <0.3, manganese: <5,and the rest iron, apart from naturally occurring impurities. 7.Compound tube according to claim 6, wherein the outer diameter of thetube is between 15 and 200 mm and that it has a total wall thickness ofbetween 2 and 20 mm.
 8. Compound tube according to claim 6, wherein thelayer of the iron chromium alloy mentioned constitutes 20-50% of thetotal wall thickness.
 9. Compound tube according to claim 6, in the formof bayonet tubing, superheater tubing, or reformer tubing.